Method for avoiding interferences in cellular CDMA radio networks caused by defective mobile stations

ABSTRACT

The invention relates to a method for avoiding interferences in cellular CDMA radio networks caused by defective mobile stations. To this end, the radio network is provided with a monitoring unit that compares an actual receive power arriving from a mobile station with a predetermined nominal receive power and initiates predetermined follow-up actions depending on the result of this comparison. Alternatively or additionally thereto, the monitoring unit is provided with at least one counter that is capable of detecting commands for power control of the mobile station that are transmitted individually for every mobile station from the base station to the mobile station and to initiate predetermined follow-up actions depending thereon.

The invention relates to methods for avoiding interference in cellularCDMA radio networks caused by defective mobile stations according to thepreamble of the independent patent claims.

In recent times, cellular radio systems using so-called code divisionmultiple access (CDMA) techniques (W-CDMA, UMTS) as multiple accessmethods are increasingly gaining in importance. In this multiple accesstechnique, the same cannier frequency is normally used for allconnections between a subscriber mobile station (terminal) and a basestation of the radio network which must be set up via the air interface,and the subscriber is individually allocated to this carrier frequencyby convolution of the baseband signal generated by the subscriber withan individual code sequence.

If signals from a number of remote stations (subscriber mobile stations)simultaneously reach a receiver of the base station, these signals arenormally all at the same carrier frequency. The demodulation of areceived code division signal essentially consists in the inverseconvolution of the received signal by means of the code sequence used inthe modulation. In this process, the signals of all other subscriberslocated in the relevant cell act as interference signals for the signalof the wanted subscriber to be demodulated. As a basic prerequisite fora satisfactory operation of the demodulator, all subscriber signalsreaching the receiver input of the base station at the same time musthave almost the same amplitude; otherwise, demodulation by inverseconvolution is not possible.

In a radio cell, the path attenuation between the mobile stations andthe base station depends on the distance between subscriber and basestation. The normal case is differences in the path attenuations of upto 80 dB between subscribers located at the edge of the cell and thebase station and, respectively, subscribers transmitting in the vicinityof the base station and the base station.

The demand for equal received field strengths at all mobile stations atthe input of the base station mandatorily leads to the introduction of acorresponding control of the respective transmit power of the mobilestations. It must be possible to decrease the transmit power of mobilestations located close to the base station by up to 83 dB. The transmitpower of the mobile stations is continuously monitored by the associatedbase station. If required; the mobile station receives from the basestation via the air interface commands for power control in order tocorrespondingly adapt its power.

In contrast to frequency-division or time-division systems (such asFDMA-Frequency Division Multiple Access or TDMA-Time Division MultipleAccess), in which each subscriber occupies a channel, the frequency ortime of which is permanently defined, on the air interface and thuscannot normally adversely affect other subscribers, each subscriber actsas an interferer for all other subscribers in this cell in CDMA systems.The subscriber generating the highest received level at the base stationdetermines the size of the radio cell in these systems. Signals fromsubscribers generating a lower received level at the base station willno longer be properly detected or decoded. Thus, there is a risk that,in CDMA systems, a subscriber with a terminal which has defectivecontrol of the transmit power, or none at all, blocks almost the entireradio cell by generating an extremely high received field strength atthe receiver of the base station. All subscribers generating a lowerreceived field strength at a base station in spite of maximum transmitpower output are suppressed (dropped call).

It is the object of the invention to propose methods by means of whichinterference caused in CDMA radio networks by defective mobile stationscan be avoided.

This object is achieved by the features of the independent patentclaims.

According to a first embodiment of the invention, it is provided that inthe radio network, a monitoring unit is provided which compares areceived power arriving from a mobile station with a predeterminednominal received power and initiates predetermined follow-up actions independence on the result of the comparison.

As a follow-up action, the interruption of the RF link between basestation and mobile station can be provided if the actual received powerexceeds, preferably exceeds for a relatively long period, the nominalreceived power. On the other hand, it can be provided that the mobilestation is completely blocked for further accesses to the radio networkif the actual received power exceeds the nominal received power a numberof times in succession.

The comparison is preferably in each case carried out during the firstuplink connection between the mobile station and a base station of theradio network.

In a second embodiment of the invention, it is provided that in theradio network, a monitoring unit is provided in which at least onecounter is set up which is capable of detecting commands for controllingthe power of the mobile station, which are sent to the mobile stationfrom the base station individually for each mobile station, and ofinitiating predetermined follow-up actions in dependence thereon.

In a preferred embodiment of the invention, the counter is incrementedby one step with each command of the base station to the mobile stationto reduce the transmit power, and is decremented by one step, or resetto zero, with each command to increase the transmit power.

As soon as the value of the counter exceeds a predetermined referencevalue, the base station interrupts the RF link associated with thecorresponding mobile station.

Before the connection is interrupted, the subscriber can be notifiedabout the reason for the interruption which is preferably transmitted inthe form of a short SMS message.

A separate counter is set up for each existing radio link between themobile station and the base station.

According to a further development of the invention, a second counter isimplemented in the monitoring unit, by means of which the connectioninterruptions initiated are counted individually for each mobilestation. If the value of the second counter exceeds a predeterminedreference value, the access of the mobile station to the radio networkis completely blocked and the subscriber is notified about the reasonfor the block before the mobile station is blocked. Here, too, thenotification can take the form of a short message (SMS).

The block of the mobile station is advantageously linked to itsinternational mobile equipment identity IMEI.

In both embodiments of the invention, it can be provided that themonitoring unit is set up in the base stations or in the base stationcontrollers of the radio network.

In the text which follows, exemplary embodiments of the invention areexplained in greater detail with reference to drawing figures. Furtherfeatures, advantages and applications of the invention are found in thedescription and in the drawings, in which:

FIG. 1: shows diagrammatically and in a simplified representation apossible device for carrying out the method;

FIG. 2: shows a flow chart of a first possible embodiment of the method;

FIG. 3: shows a flow chart of a second possible embodiment of themethod.

In the example according to FIG. 1, the CDMA radio network considered,comprises at least one base station 1 and one mobile station 10 whichcan exchange radio signals with one another via an air interface 9.

The base station 1 comprises a transceiver unit 2 and is controlled by abase station controller 8 which can be arranged inside the base station1 or also externally.

In the base station 1 (or in the base station controller 8), amonitoring unit 3 is set up which essentially comprises an evaluatinglogic 4, a reference value memory 5 for storing a predetermined nominalreceived power (reference received power) and a first and a secondcounter 6 and 7, respectively. The monitoring unit 3 is connected to thetransceiver device 2 and to the base station controller 8 and canexchange (control) signals with these devices.

In a first embodiment of the invention, the received power of the mobilestation 10 arriving at the receiver 2 of the base station 1 is evaluatedin the monitoring unit 3.

Normally, the mobile station 10, when registering in the radio network,generates an uplink received level at the receiver input 2 of the basestation 1 which approximately corresponds to the expected nominal inputlevel. The reason for this behavior that the mobile station 10 monitorsthe pilot carrier of the base station 1 in its standby mode and sets itsoutput power in accordance with the path attenuation assumed as aresult. This type of power control is also called open loop powercontrol.

If the power control of the mobile station 10 is defective, it mayregister with much too high an uplink signal level at the base station1. If the uplink signal level for a subscriber link exceeds a nominalthreshold value predetermined by the network operator several times insuccession, it must be assumed that the power control of the relevantmobile station 10 is defective.

This method is illustrated in FIG. 2. The monitoring unit 3 monitors thesignal level of the mobile station 10 at the input of the receiver 2 ofthe base station 1 (step 20). A comparison is made regularly between theinput signal level received by the mobile station 10 and a predeterminednominal signal level (step 21) which is stored in the reference valuememory 5. If the input signal level is less than the predeterminednominal signal level, a regular connection is set up to the mobilestation 10.

If, in contrast, the input signal level is greater than thepredetermined nominal signal level, the value of the counter 6 (Z1) isincremented by one step (step 22).

The value of the counter 6 is then compared with a predeterminedreference value (step 23). If the value of the counter 6 is less thanthe reference value, the connection set-up is continued.

If, in contrast, the value of the counter 6 is greater than thepredetermined reference value, the connection between mobile station 10and base station 1 is interrupted and the mobile station is firstnotified about the reason for the interruption to the connection, e.g.by SMS (steps 25 and 24).

A second embodiment of the invention is described with reference toFIGS. 1 and 3. In this embodiment, the monitoring unit 3 detects thecommands to the mobile station 10 for power control (step 30), comingfrom the base station controller 8 in an existing radio link betweenmobile station 10 and base station 1. The counter 6 (Z1) is used forcounting and evaluating the commands to the mobile station for powercontrol, which are sent from the base station to the mobile station.

If the monitoring unit 3 detects a command which contains a request forincreasing the transmit power of the mobile station (step 31), the valueof the counter 6 is reset to zero (step 32).

If the monitoring unit detects a command for decreasing power to themobile station 10 (step 33), the value of the counter 6 is incrementedby one step (step 34).

A comparison is then made as to whether the value of the counter 6exceeds a predetermined reference value (step 35). The reference valueis stored in the reference value memory 5.

If this is not so, the process continues with step 30.

If the value of the counter 6 exceeds the reference value, the value ofa second counter 7 is incremented by one step (step 36). This counter 2counts the number of reference value transgressions of the first counter6 which is equivalent to the connection interruptions which havepreviously occurred for the relevant mobile station 10. A comparison isthen made as to whether the value of the second counter 7 exceeds apredetermined reference value (step 37) which is also stored in thereference value memory 5.

If this is not so the process continues with step 38 whereupon the RFlink associated with the relevant mobile station 10 is interrupted (step39). In addition, the subscriber can be correspondingly notified aboutthe reason for the interruption (step 38) before the radio link isinterrupted.

If the value of the second counter 7 exceeds the predetermined referencevalue, the access of the mobile station 10 to the radio network iscompletely blocked (step 41) since it can be assumed that the powercontrol of the mobile station is defective. First, the mobile stationcan be notified about the complete block (step 40).

A combination of the two evaluating methods can be useful. The firstmentioned method prevents interference by terminals with failed powercontrol whereas the second method described can also recognize thedetection of terminals with wrongly implemented RF power control (e.g.control too slow).

KEY TO THE DRAWINGS

-   1 Base station-   2 Transceiver-   3 Monitoring unit-   4 Logic-   5 Reference value memory-   6 First counter (Z1)-   7 Second counter (Z2)-   8 Base station controller-   9 Air interface-   10 Mobile station-   20 Detect received power-   21 Compare received power-   22 Increment counter Z1-   23 Compare counter Z1-   24 Notify MS-   25 Interrupt connection-   30 Detect commands-   31 Command for increasing power-   32 Set Z1 to zero-   33 Command to reduce power-   34 Increment counter Z1-   35 Compare counter Z1-   36 Increment counter Z2-   37 Compare counter Z2-   38 Notify MS-   39 Interrupt connection-   40 Notify MS-   41 Block MS

1. A method for avoiding interference in cellular CDMA radio networkscaused by defective terminals, comprising providing at least one counterin a monitoring unit set up so as to be capable of detecting commandsfor controlling the power of a mobile stations which are sent from abase station to the mobile station individually for each mobile station,and of initiating predetermined follow-up actions in dependence thereon,wherein the base station interrupts an RF link associated with thecorresponding mobile station if the value of the counter exceeds apredetermined reference value; providing a second counter for countingindividually the connection interruptions initiated for each mobilestation; and blocking the access of the mobile station to the radionetwork completely if the value of the second counter exceeds apredetermined reference value.
 2. The method as claimed in claim 1,wherein the counter is incremented by one step with each command of thebase station to the mobile station to reduce the transmit power, and isdecremented by one step, or reset to zero, with each command to increasethe transmit power.
 3. The method as claimed in claim 1, wherein themobile station is notified about the reason for the interruption beforethe connection is interrupted.
 4. The method as claimed in claim 1,wherein notification is in the form of a short SMS message.
 5. Themethod as claimed in claim 1, wherein a separate counter is set up foreach existing radio link between the mobile station and the basestation.
 6. The method as claimed in claim 1, wherein the monitoringunit compares an actual received power arriving from a mobile stationwith a predetermined nominal received power and initiates predeterminedfollow-up actions in dependence on the result of the comparison.
 7. Themethod as claimed in claim 6, wherein as follow-up action, the RF linkbetween the base station and mobile station is interrupted if the actualreceived power exceeds the nominal received power.
 8. The method asclaimed in claim 6, wherein as follow-up action, the mobile station isblocked for further accesses to the radio network if the actual receivedpower exceeds the nominal received power a number of times insuccession.
 9. The method as claimed in claim 6, wherein the comparisonis in each case carried out during the first uplink connection betweenthe mobile station and a base station of the radio network.
 10. Themethod as claimed in claim 6, wherein before the mobile station isblocked, the subscriber is notified about the reason for the block. 11.The method as claimed in claim 10, wherein the notification is in theform of a short SMS message.
 12. The method as claimed in claim 1,wherein the block of the mobile station is linked to its internationalmobile equipment identity (IMEI).
 13. The method as claimed in claim 1,wherein the monitoring unit is set up in the base stations of the radionetwork.
 14. The method as claimed in claim 1, wherein the monitoringunit is set up in the base station controllers of the radio network.